Here we will interface Bluetooth module HC 05. Bluetooth communication occurs between a master BT device and a slave BT device. Bluetooth devices are symmetric, means; the same device may operate as a master and also the slave. Each BT device has a 48-bit unique device address (BD_ADDR) that is fixed.

Bluetooth hc05 component

Architecture of these modules is based on the Cambridge Silicon Radio BC417, 2.4 GHz Bluetooth Radio chip, as they are exchanging data by the waves are in range of radio frequency. Chip uses an external 8 MB flash memory. For more detail information goes through its data sheet.


  1. Typical ‐80dBm sensitivity.
  2. Up to +4dBm RF transmits power.
  3. 3 to 5 V I/O.
  4. PIO (Programmable Input/Output) control.
  5. UART interface with programmable baud rate.
  6. With integrated antenna.
  7. With edge connector.


The HC-05 Bluetooth Module has 6 pins commonly used. They are as follows:


When enable is pulled LOW, the module is inactivated which means the module will not turn ON and it fails to transmit/receive data (communicate). When enable is left open or connected to 3.3V, the module is enabled i.e. the module remains on and communication also takes place.

2)  VCC

Supply Voltage 3.3V to 5V

3) GND

Ground pin connected to ground.

Next these two pins act as an UART interface for communication

4) TXD:

To transmit data from BT memory to device (generally microcontroller or microprocessor)

5) RXD

To receive data from device to BT memory


It acts as a status indicator. When the module is not connected to/paired with any other Bluetooth device, signal goes Low. At this low state, the led (inbuilt in module) flashes continuously which denotes that the module is not paired with another device. When this module is connected to/paired with any other Bluetooth device, the signal goes High. At this high state, the led blinks with a constant delay say for example 2s delay which indicates that the module is paired.

One more important thing to notice in module is small button.


This is used to switch the module into AT command mode.

What is AT command mode? Where we can change or see its default parameter like its name, password etc. Actually there are two modes for such devices one is command mode another is data mode, when it is in command mode any instruction sent to it are interrupts as command. Here commands are preceded by letters “AT” (stands for attention).

Bluetooth Module HC 05

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